Ultrasound Imaging Apparatus

The following generally relates to ultrasound and more particularly to an ultrasound imaging apparatus, to a corresponding method, data processing system and computer program.

High-frame-rate 3-D imaging with a broad volumetric coverage can be achieved using 2-D probes combined with synthetic aperture imaging. However, for most 2-D probe designs, the number of channels increases quadratically with the number of elements in the side-length of aperture. In practice, this makes it very difficult, if not impossible, from a manufacturing and processing standpoint to achieve a low F-number at large imaging depths. Furthermore, even small numbers of elements in the side-length of aperture result in a channel count far exceeding that of typical 2-D imaging apparatus.

Row-column addressed arrays (RCAs) provide a solution to the high channel count by addressing the elements of the 2-D array by rows and columns, and this reduces the total number of channels to scale linearly, rather than quadratically, with the number of elements in the side-length of aperture.

U.S. Pat. No. 10,705,210 discloses an ultrasound imaging apparatus for three-dimensional imaging with a row-column addressed transducer array using synthetic aperture sequential beamforming. This prior art method applies fixed focusing to simplify the propagation paths of the soundwaves.

Row-column imaging with dynamic receive focusing is another approach to row-column imaging. Dynamic receive focusing models the sound waves propagation more accurately and most current ultrasound scanners use this approach in the imaging.

Row-column imaging with dynamic receive focusing is described in M. F. Rasmussen, T. L. Christiansen, E. V. Thomsen, and J. A. Jensen, “3-D imaging using row-column-addressed arrays with integrated apodization—Part I: Apodization design and line element beamforming,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., vol. 62, no. 5, pp. 947-958, 2015.

L.Th. Jørgensen et al., “Tensor Velocity Imaging With Motion Correction”, IEEE Transactions on ultrasonics, ferroelectrics, and frequency control, Vol. 66, No. 5, May 2021 presents a motion compensation procedure for improving the accuracy of synthetic aperture tensor velocity estimates for row-column arrays.

While row-column imaging with dynamic receive focusing has been found to provide a high image quality, this approach places a high demand on the processing unit. This is a particularly severe issue when volumetric images are to be acquired, as these usually contain a large number of image points. In particular, the high processing demands severely limit the pulse repetition frequency at which real-time imaging can be performed. However, many clinical applications, such as real-time flow applications, require high pulse repetition rates.

It thus remains desirable to provide an imaging apparatus that facilitates high pulse repetition frequencies such as for real-time volumetric beamforming while providing a high image quality.

In view of at least the foregoing, there is an unresolved need for an improved approach to row-column ultrasound imaging.

Various aspects disclosed herein seek to address the above and/or other matters or at least seek to provide an approach that may serve as an alternative to existing approaches.

According to one aspect, an ultrasound imaging apparatus is disclosed for providing a volumetric ultrasound image of an image volume. The term volumetric ultrasound image is intended to refer to a 3-D ultrasound image, which may be represented as a 3-D raster of image points, e.g. image points each having an image value associated with it.

Embodiments of the ultrasound imaging apparatus comprise:

The row-column addressed transducer array is configured to convert excitation electrical pulses into an ultrasound pressure field and to convert received ultrasound echo pressure fields into echo signals. The beamformer module is configured to beamform the echo signals using dynamic receive focusing to produce respective image values at a first set of image points within the image volume.

The reconstruction module is configured to:

The ultrasound imaging apparatus may be configured to control the row-column addressed transducer array to make a plurality of ultrasonic emissions corresponding to ultrasound emitted from respective virtual emitter locations; wherein the beamformer module and the reconstruction module are configured to compute a plurality of low-resolution volumetric images, each low-resolution volumetric image corresponding to a respective virtual emitter location, and wherein the image apparatus further comprises an image combiner module configured to combine the plurality of low-resolution volumetric images corresponding to different virtual emitter locations into a combined high-resolution volumetric image having a spatial resolution higher that the low-resolution volumetric images.

Embodiments of the ultrasound imaging apparatus disclosed herein are capable of providing volumetric images that are indistinguishable from prior art row-column imaging methods with dynamic receive focusing, but with considerably fewer operations.

In prior art row-column imaging using dynamic receive focusing, the number of processing operations is proportional to the number of elements of the array times the number of image points. In embodiments of the approach used in the various aspects disclosed herein, the number of operations is proportional to just the number of image points. The inventors have realized that this reduction can be achieved because there are positions in the volume where the measured sound's time of flight to a closest position of the aperture is constant. As a result, the image value along these positions will be approximately constant as well. This means that imaging apparatus only needs to calculate the image value in one of those positions to get the image values for all the remaining positions. Various embodiments disclosed herein take advantage of this realization in 3-D imaging, thereby achieving a significant reduction in the number of processing operations, thereby facilitating real-time image acquisition at a higher pulse frequency. Real-time image acquisition at a high pulse frequency in turn facilitates a variety of applications, such as for clinical use. Examples of such applications include, but are not limited to anatomic imaging, super resolution imaging, contrast imaging, and velocity imaging, such as tensor velocity imaging.

The row-column addressed transducer array includes a plurality of transducer elements. The transducer elements may be configured to convert excitation electrical pulses into an ultrasound pressure field and to convert received ultrasound pressure fields (echoes) into an electrical (e.g., a radio frequency (RF)) echo signal. The echoes and, hence, the echo signals are generated in response to the transmitted pressure field interacting with matter, e.g., tissue, etc. The ultrasound imaging apparatus may thus comprise a transmit circuit configured to generate the excitation electrical pulses, in particular so as to cause the row-column addressed transducer array to an emission sequence of ultrasound pressure fields corresponding to respective virtual emitter locations.

The row-column transducer array may include a first set of transducer elements defining a first transducer array, in particular a first 1-D transducer array, along a first axis. Each transducer of the first transducer array may be elongated along a first longitudinal axis which may be orthogonal to the first axis. To this end, the transducers of the first transducer array may be formed as rows or columns of a 2-D array of transducer elements, where a single channel may be used to address the transducer elements of each row or column. Each column or each row may thus be addressable as a single elongated transducer of a 1-D array of transducers. The row-column transducer array may include a second set of transducer elements defining a second transducer array, in particular a second 1-D transducer array, along a second axis. Each transducer of the second transducer array may be elongated at a second longitudinal axis, which may be orthogonal to the second axis. To this end the transducers of the second transducer array may be formed as columns or rows of a 2-D array of transducer elements, where a single channel may be used to address the transducer elements of each column or row. The first axis may be orthogonal to the second axis. In particular, the row-column addressed array may define two elongated 1-D transducer arrays, one consisting of the row elements and another consisting of the column elements of a 2-D array of transducer elements.

In some embodiments, the imaging apparatus is configured to transmit ultrasound waves by the first transducer array and to receive backscattered ultrasound waves, i.e. echoes, by the second transducer array.

The reconstruction module determines trajectories in the image volume. This determination may involve determination of trajectories along which the image values are constant or at least approximately constant. To this end, the reconstruction module may determine a trajectory as a set of positions within the image volume where a time-of-flight from a virtual emitter location, via any one of the set of positions along the trajectory, to a closest position on an aperture of a receiving one of the first and second transducer arrays is constant along said trajectory, i.e. is the same for all positions of the set of position. Accordingly, the set of trajectories are trajectories of constant time of flight in respect of a virtual emitter location. The determination of the trajectories depends on the virtual emitter locations. Accordingly, the determination may be made for a set of virtual emitter locations, resulting in a set of trajectories, each trajectory of the set corresponding to a different virtual emitter location. A representation of the resulting trajectories, and/or a representation of corresponding mappings of image coordinates of the second set of image points along a respective one of the determined trajectories onto the first set of image points, may be stored in memory and re-used for the creation of subsequent volumetric ultrasound images that utilize the same virtual emitter locations. To this end, in some embodiments, the reconstruction module is configured to map each image coordinate of the image volume onto an image point of the first set, and to store a representation of the mapped image coordinates. Accordingly, the reconstruction module may be configured to compute a plurality of volumetric images using the stored representation of the mapped image coordinates.

A volumetric ultrasound image obtained by means of an RCA based on echo signals corresponding to a single virtual emitter location provides limited spatial resolution, in particular along a direction of elongation of the receiving transducer elements. In order to increase spatial resolution, the imaging apparatus may be configured to control the row-column addressed transducer array to make an emission sequence including a plurality of ultrasonic emissions corresponding to ultrasound emitted from respective virtual emitter locations. The virtual emitter locations may be distributed the direction of elongation of the individual receiving transducer elements and/or distributed at different distances from the array. The beamformer module and the reconstruction module may thus be configured to compute a plurality of low-resolution volumetric images (LRVs), each low-resolution volumetric image corresponding to a respective virtual emitter location. The image apparatus may further comprise an image combiner module configured to combine the plurality of low-resolution volumetric images corresponding to different virtual emitter locations into a combined high-resolution volumetric image (HRV) having a spatial resolution which, at least along one spatial direction, is higher that the corresponding resolution of the low-resolution volumetric images. Combining may include computing image values of the high-resolution volumetric image as sums, or weighted sums, of image values at the same image point of the respective low-resolution volumetric image.

Various embodiments of the method disclosed herein thus perform dynamic receive focusing along all three spatial axes. In particular, dynamic receive focusing along the direction of elongation of the receiving transducer elements, i.e. the second axis of elongation, may be achieved by combining low-resolution volumetric images acquired for different virtual emitter locations.

To this end, the beamformer module may be configured to beamform echo signals provided by the transducer array in response to ultrasound echoes received by the row-column addressed transducer array responsive to the emissions to produce a corresponding plurality of two-dimensional images, each two-dimensional image corresponding to one of the virtual emitter locations. The reconstruction module may then be configured to compute at least a first low-resolution volumetric image of the plurality of low-resolution volumetric images, the first low-resolution image corresponding to a first virtual emitter location. To this end, the reconstruction module may be configured to perform an interpolation. The construction module may compute at least the first low-resolution volumetric image by at least:

The beamformer module may be configured to perform delay-and-sum beamforming to compute the image values at the first set of image points. Beamforming the echo signals using dynamic receive focusing generally comprises applying respective delays to the responses of the individual receiving transducer elements originating from the image point, and coherently adding these delayed responses. The delays are found from the round trip time-of-flight (TOF), which is the propagation time of the emitted wave from the transmit origin, i.e. from the virtual source location, to the image point and return to one of the transducer elements of the receiving transducer array. Accordingly, beamforming may comprise computing a time-of-flight for each virtual emitter location and for each image point of the first set of image points. In particular, a new set of delay values are, thus, calculated for each image point. Hence, beamforming the echo signals using dynamic receive focusing comprises computing a time-of-flight along a shortest path from the virtual emitter location to the image point and further from the image point to a receiving transducer element, or receiving aperture, of the row-column addressed transducer array, e.g. as described in M. F. Rasmussen, T. L. Christiansen, E. V. Thomsen, and J. A. Jensen, “3-D imaging using row-column-addressed arrays with integrated apodization—Part I: Apodization design and line element beamforming,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., vol. 62, no. 5, pp. 947-958, 2015 or in Stuart et al., “Real-time volumetric synthetic aperture software beamforming of row-column probe data,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., Apr. 8, 2021, the time of flight associated with a virtual emitter location and with an image point may be computed from a path length of a shortest path extending from the virtual emitter location to the image point and from the image point to a receiving transducer element of the second transducer array, in particular to a position closest to the image point, along the elongated receiving transducer element.

The first set of image points may define a two-dimensional image surface, preferably an image plane, in the image volume, i.e. all image points of the first set of image points may lie on the two-dimensional image surface. Accordingly, the image values of the image points of the first set of image points may represent a two-dimensional image. The two-dimensional image may be considered as a 2-D projection of the image volume onto the two-dimensional image surface, the projection being defined by the trajectories. The two-dimensional image surface may be chosen such that all image points in the image volume may be mapped, in particular projected, onto the two-dimensional image surface by the set of trajectories. The second set of image points includes image points displaced from the two-dimensional image surface. It will be appreciated that the second set of image positions may further include the first set of image points, which may thus be considered being mapped onto themselves. Accordingly, the image values at the second set of image points represent a volumetric ultrasound image of the image volume.

The image plane may extend orthogonally to a longitudinal direction of the receiving transducer elements. The image plane may extend out of, in particular orthogonally, the plane defined by the transducer array. In one embodiment, the position of the image plane along the longitudinal direction of the receiving transducer elements corresponds to, in particular is equal to, the position of the virtual emitter location along said longitudinal direction of the receiving transducer elements.

The beamformer module may be configured to compute image values at image points distributed across the two-dimensional image surface, in particular the image plane, at a suitable, e.g. predetermined, sampling rate or raster density. The sampling rate may be uniform or it may vary across the image plane, e.g. may be larger along one direction than the other. In some embodiments, the ultrasound imaging apparatus is configured to sample the beamformed image plane, at least along a direction extending out of a plane defined by the transducer array, at at least the Nyquist frequency, thereby facilitating accurate interpolation by the reconstruction module.

The apparatus may include one or more processing units programmed or otherwise configured to implement the beamformer module and/or the reconstruction module and/or the combiner module. The one or more processing units may include a central processing unit (CPU), a graphics processing unit (GPU), a microprocessor, and/or the like. The beamformer module and the reconstruction module may be implemented by the same processing unit or by separate processing units.

In some embodiments, the imaging apparatus includes a probe and a console operatively coupled to the probe. The probe includes the row-column addressed transducer array and the console may include the processing unit implementing the beamformer module and/or the reconstruction module and/or the combiner module. In some embodiments some of the signal processing, e.g. some or all of the beamforming operation may be performed by a processing unit included in the probe.

In some embodiments, the imaging apparatus includes transmit circuitry configured to generate the excitation electrical pulses that excite the transducer elements and receive circuitry configured to receive and, optionally, condition and/or preprocess the electrical, e.g. RF, signals produced by the transducer elements. The transmit and/or receipt circuitry may be included in the console.

In some embodiments, the processing unit is configured to implement a processing pipeline that processes the received electrical signals, in particular the received RF signals, to create a representation of one or more volumetric ultrasound images.

The same and/or a separate processing unit may be configured to implement additional processing of the one or more of volumetric ultrasound images, e.g. for velocity imaging and/or another type of 3-D ultrasound imaging.

It will be appreciated that the creation of the one or more ultrasound images from the received electrical signals and the optional further processing may be implemented by a single processing unit. Alternatively, the different acts may be distributed between multiple processing units. In some embodiments, the processing unit(s) implementing the signal and/or image processing are included in the console. In some embodiments, at least some of the signal processing and/or subsequent image processing may be implemented by a computing apparatus separate from the console. With this embodiment, RF signals stored in memory and/or beamformed images stored in memory of the computing apparatus or received from the console (e.g. via a suitable wired or wireless connection) can be loaded and processed by an image processing pipeline to generate the resulting 3-D images.

The imaging apparatus may further include a display and be configured to display a representation of the generated volumetric images. The display may be included in the console or in a separate computing apparatus.

In another aspect, a method, in particular a computer-implemented method, includes:

In yet another aspect, a computer program includes instructions that when, executed by a computer or other data processing system, cause the computer or other data processing system to perform the computer-implemented acts of the method described herein. The computer program may be implemented as a computer-readable storage medium storing the instructions or as a data signal encoding the instructions.

According to another aspect, disclosed herein are embodiments of a data processing system configured to perform the acts of the method described herein. In particular, the data processing system may have stored thereon program code adapted to cause, when executed by the data processing system, the data processing system to perform the steps of the method described herein. The data processing system may be embodied as a single computer or as a distributed system including multiple computers, e.g. a client-server system, a cloud based system, etc.

Those skilled in the art will recognize still other aspects of the present application upon reading and understanding the attached description.

The following describes an ultrasound imaging apparatus and associated method for providing a volumetric ultrasound image of an image volume and a corresponding method that mitigate one or more of the above-noted shortcomings of prior art volumetric imaging approaches. Generally, the ultrasound imaging apparatus comprises:

illustrates an example imaging apparatusconfigured for volumetric ultrasound imaging of a subject. The imaging apparatusincludes a probeand a console, which interface with each other through suitable complementary hardware (e.g., electromechanical connectorsandand a cableas shown, etc.) and/or a wireless interface (not visible).

The probeincludes a row-column addressed transducer array (RCA)with a plurality of transducer elements. The transducer arrayincludes a planar, curved or otherwise shaped, a fully populated or sparse, etc. array. The transducer elementsare configured to convert excitation electrical pulses into an ultrasound pressure field and to convert received ultrasound pressure fields (echoes) into an electrical (e.g., a radio frequency (RF)) echo signal. The echo signals are thus generated in response to the transmitted pressure field interacting with matter, e.g., human tissue other biological or non-biological material, etc. The transducer elements of the RCA are arranged in a 2-D array.

The consoleincludes transmit circuitry (TX)configured to generate the excitation electrical pulses that excite the transducer elementsand receive circuitry (RX)configured to receive the RF signals produced by the transducer elements. In one embodiment, the RX(or other circuitry) is configured to also condition or preprocess the RF signal, e.g., amplify, digitize, etc. In the illustrated embodiment, a TX/RX controlleris configured to control the TXand RXfor transmit and receive operations of the RCA. The TXand RXaddress the transducer elements of the RCA by row and columns, respectively. To this end, the signal received along the rows or columns is summed to create one signal per row or column. The RCAthus effectively forms two orthogonal 1-D arrays with elongated transducer elements, e.g. as described in connection with.

In one embodiment, the TXand RXare controlled to cause the RCA to perform volumetric imaging by transmitting with one of the 1-D arrays and receiving the backscattered signal with the other 1-D array or with the same 1-D array. For example, a subset (i.e. one or a subgroup) of the elements of the transmitting array can be excited to simultaneously produce pressure fields that together emit a focused beam corresponding to an emission from a virtual emitter location, in particular a virtual line emitter. All or a subset of the elements of the receiving array can be used to receive echoes. This can be repeated for multiple different subsets of emitting arrays and, in particular, for different locations of the virtual emitter, where each emission/reception provides data to generate a low-resolution volumetric image, and a high-resolution volumetric image can be generated by combining multiple low-resolution images corresponding to different virtual emitter locations as described herein. An example of a suitable sequence is described in Jensen et al., “Synthetic aperture ultrasound imaging,” Ultrasonics, vol. 44, pp. e5-e15, 2006. Another example using plane waves is mentioned in Tanter et al., “Ultrafast imaging in biomedical ultrasound, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2014, 61, 1, pp. 102-119.

The consolefurther includes a processing unit. The processing unitcan include one or more processors (e.g., a central processing unit (CPU), graphics processing unit (GPU), a microprocessor, etc.) configured to execute computer-readable instructions encoded or embedded on a computer-readable storage medium such as memoryto perform the computer-implemented acts described herein. In general, the processing unitis configured to process the RF signals to create a set of low-resolution volumetric ultrasound images and to combine the low-resolution volumetric ultrasound images to generate a high-resolution volumetric ultrasound image. In particular, the processing unitis configured to implement a processing pipeline including a beamformer module, a reconstruction module and a combiner module as described herein.

As described in greater detail below, in one instance the processing unitimplements a beamformer module, a reconstruction module and a combiner module. The beamformer module beamforms the received echo signals so as to produce beamformed low-resolution image planes, which correspond to different virtual emitter locations. The reconstruction module computes a low-resolution volumetric image from each of the beamformed low-resolution image planes. To this end, the reconstruction module determines, for each low-resolution image, trajectories of at least approximately constant image values, the trajectories intersecting the beamformed low-resolution image plane, maps image coordinates of the image volume onto the beamformed low-resolution image plane using the determined trajectories, and to interpolate the image values at the mapped image coordinates from the image values of the beamformed low-resolution image plane, thereby obtaining a low-resolution volumetric ultrasound image. The combiner module combines the resulting plurality of low-resolution volumetric ultrasound images corresponding to different virtual emitter locations into a combined high-resolution volumetric ultrasound image.

The consolemay further include a scan converterand a display. The scan converteris configured to scan convert each image for display, e.g., by converting the images to the coordinate system of the display. A representation of a high-resolution volumetric ultrasound image is then displayed, e.g. a 3D rendering, a cross-section or another type of representation. Alternatively or additionally to displaying a representation of the generated high-resolution volumetric ultrasound image, the high-resolution volumetric ultrasound image may be stored and/or further processed, e.g. so as to identify and/or classify structures or properties of the subject being imaged.

The consolefurther includes a user interface, which includes one or more input devices (e.g., a button, a touch pad, a touch screen, etc.) and one or more output devices (e.g., a display screen, a speaker, etc.). The consolefurther includes a controllerconfigured to control one or more of the transmit circuitry, the receive circuitry, the TX/RX controller, the processing pipeline, the scan converter, the display, and/or the user interface.

It will be appreciated that other embodiments of an imaging apparatus may include alternative or additional components and/or the components may be arranged in a different manner and/or some components may be omitted. For example, the distribution of components between the probe and the console may be different, or the apparatus may include an additional remote data processing system.